System and method for dispensing liquid from storage containers

ABSTRACT

A system for dispensing liquid from first and second containers is disclosed herein. The liquid dispensing system includes a first media transfer line, positioned in liquid-flow communication with the first container, for transferring liquid from the first container. A second media transfer line, positioned in liquid-flow communication with the second container, is disposed to transfer liquid from the second container. When placed in a first state a switchable valve prevents liquid from flowing through the first media transfer line and allows liquid to flow through the second media transfer line. In a second state the switchable valve prevents liquid from flowing through the second media transfer line while allowing liquid to flow through the first media transfer line. A liquid storage reservoir connected in liquid-flow communication with the switchable valve stores liquid received from the first and second media transfer lines by way of the switchable valve. A transfer line sensor arrangement monitors the first and second media transfer lines for the presence of liquid therein, and generates a first switching signal when liquid becomes absent from the first media transfer line and a second switching signal when liquid becomes absent from the second media transfer line. The switchable valve is switched to the first state in response to the first switching signal and to the second state in response to the second switching signal. A vacuum generator is disposed to remove gas from the liquid storage reservoir above the level of liquid therein.

The present invention relates generally to systems for dispensingliquids such as liquid chemicals, and particularly to systems disposedto automatically switch between at least a pair of liquid sourcecontainers.

BACKGROUND OF THE INVENTION

As is well known, large numbers of integrated circuit elements are oftenfabricated on a single semiconductor substrate wafer. In order tomaximize the yield of the wafer, i.e., the percentage of non-defectivecircuit elements, it is essential to precisely control the processingenvironment. This is of particular importance during photolithographicprocessing. As an initial step in such processing the wafer is coatedwith a photosensitive layer using a liquid chemical such as photoresist.A desired circuit pattern is then defined by:

(i) selectively exposing the photosensitive layer by illuminating a maskusing light of a specific wavelength and intensity, and

(ii) removing unexposed portions of the photosensitive layer using, forexample, chemical etchants.

Since the portion of an integrated circuit underlying a coated area ofthe wafer can be destroyed if exposed to chemical etchants, acceptablewafer yields require that the coated area be virtually free ofdiscontinuities. Such discontinuities are known to arise, for example,as a consequence of air bubbles present in photoresist dispensingsystems. Hence, wafers tend to be destroyed, and wafer yields therebydegraded, in proportion to the relative amount of air present within thedispensed photoresist.

In manual dispensing systems the photoresist liquid is generally drawnfrom a small bottle or container which must be replaced when empty.However, during manual replacement of the photoresist container there isa tendency for air to become trapped in various supply lines of thedispensing system. That is, if a container becomes completely emptybefore a change is made air will be introduced into the dispensingsystem. This can cause discontinuities in the photoresist layer, andrequires that a purging procedure be performed to rid the system of theunwanted air. Such a purging operation wastes residual photoresistpresent within the dispensing lines and reduces production efficiency.In addition, unwanted air within the dispensing system can result inparticle formation due to oxidation of the surface of the photoresist.As is well known, the presence of such particles within the photoresistcan create defects or discontinuities in the circuit lines defined onthe wafer.

As photoresist and other processing liquids have become more expensive,it has become apparent that the waste of processing liquids duringmanual dispensing operations can significantly increase the cost ofwafer processing.

OBJECTS OF THE INVENTION

One object of the present invention is to provide a liquid dispensingsystem which reduces the possibility of ambient air surrounding thesystem being introduced therein.

Another object of the present invention is to provide a system fordispensing liquid chemicals capable of automatically switching betweensupply containers in a manner which minimizes waste of such chemicals,and which reduces the possibility of air becoming entrapped therein.

Yet another object of the present invention is to provide a system fordispensing an uninterrupted supply of liquid chemicals substantiallyuncontaminated by ambient air.

SUMMARY OF THE INVENTION

In summary, the present invention is a system for dispensing liquid fromfirst and second containers. The liquid dispensing system of the presentinvention includes a first media transfer line, positioned inliquid-flow communication with the first container, for transferringliquid from the first container. A second media transfer line,positioned in liquid-flow communication with the second container, isdisposed to transfer liquid from the second container. When placed in afirst state a switchable valve prevents liquid from flowing through thefirst media transfer line and allows liquid to flow through the secondmedia transfer line. In a second state the switchable valve preventsliquid from flowing through the second media transfer line whileallowing liquid to flow through the first media transfer line. A liquidstorage reservoir connected in liquid-flow communication with theswitchable valve stores liquid received from the first and second mediatransfer lines by way of the switchable valve. A transfer line sensorarrangement monitors the first and second media transfer lines for thepresence of liquid therein, and generates a first switching signal whenliquid becomes absent from the first media transfer line and a secondswitching signal when liquid becomes absent from the second mediatransfer line. The switchable valve is switched to the first state inresponse to the first switching signal and to the second state inresponse to the second switching signal. A vacuum generator is disposedto remove gas from the liquid storage reservoir above the level ofliquid therein.

BRIEF DESCRIPTION OF THE DRAWINGS

Additional objects and features of the invention will be more readilyapparent from the following detailed description and appended claimswhen taken in conjunction with the drawings, in which:

FIG. 1 is a block diagrammatic representation of a preferred embodimentof a system for dispensing liquids, such as semiconductor processingchemicals, from first and second liquid media containers.

FIG. 2 shows a cross-sectional side view of a liquid reservoir includedwithin the dispensing system of the invention.

FIGS. 3A-3D are a set of flow charts outlining the procedure by which aliquid reservoir included within the dispensing system of FIG. 1 isrefilled in accordance with the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIG. 1, there is shown a preferred embodiment of a system10 for dispensing liquids, such as semiconductor processing chemicals,from first and second liquid media containers 14 and 18. The system 10is operative to dispense liquid from either the first container 14 orsecond container 18 until the liquid therein is depleted, and to thenautomatically switch to dispensing liquid from the other. Liquid iswithdrawn from the first container 14 via a first media supply line 22in liquid-flow communication with a first valve 26, while a second mediasupply line 30 in liquid-flow communication with a second valve 34 isdisposed to withdraw liquid from the second media container 18. In thepreferred embodiment of FIG. 1 the valves 26 and 34 arepneumatically-controlled, and serve to selectively permit liquid to flowfrom the containers 14 and 18 to a liquid reservoir 40.

More specifically, when liquid is being withdrawn from the firstcontainer 14 the second valve 34 operates to prevent liquid from flowingthrough the second media supply line 30. In this way the secondcontainer 18 may be replenished with liquid without interrupting theflow of liquid to the reservoir 40. Likewise, when the first container14 becomes empty the first valve 26 is automatically closed, and thesecond valve 34 contemporaneously opened, so as to allow the liquidwithin the first container 14 to be replenished. The valves 26 and 34may be implemented using, for example, 2-way pneumatic valves of thetype manufactured by Fluoware Inc., of Chasua Minnesota, Part No.PV2-1144.

As is described more fully below, first and second optical flow-throughsensors 44 and 48 are disposed to electronically signal when thecontainers 14 and 18 become empty by monitoring the media supply lines22 and 30, respectively, for the presence of photoresist. That is, thepresence of air within one of the media supply lines indicates that thecorresponding container 14, 18 in liquid-flow communication therewith isempty, In this way the valves 26 and 34 may be controlled so as toprevent air from reaching the reservoir 40 during switching between thecontainers 14 and 18.

Each of the optical sensors 44 and 48 will preferably include, forexample, a photometric device such as a Photomicrosensor, Part No.EE-SPX403, manufactured by Omron Electronics Inc., of Schaumburg,Illinois, capable of discerning the presence of opaque liquid materialwithin the supply line or conduit being monitored. In the preferredembodiment the photometric device is mounted within a housing, throughwhich also extends the liquid supply line or conduit being monitored. Inthis implementation the liquid supply lines or conduits are realizedusing optically transparent tubing, e.g., Teflon tubing, so as to enablelight emitted by the photometric device to interact with liquid presenttherein. Hence, each of the supply lines or conduits may be monitoredwithout interfering with the flow liquid through the system 10.

In the preferred embodiment of FIG. 1 the containers 14 and 18 arelight-blocking glass or plastic containers suitable for storingsemiconductor processing chemicals, such as photoresist. As shown inFIG. 1, media supply lines 22 and 30 are directed by guide tubeassemblies 49 and 50 to extend proximate the bottom of the containers 14and 18, respectively, so as to withdraw liquid from the lower portionsthereof. In a first operative state of system 10, during which the firstvalve 26 is opened and the second valve 34 is closed, liquid istransferred from the first container 14 via supply line 22 and areservoir feed line 52 to the reservoir 40. Likewise, in a secondoperative state liquid is carried from the second container 18 toreservoir 40 through valve 34 via the media transfer line 30 andreservoir feed line 52.

Referring to FIG. 1, pneumatic control of the valves 26 and 34 iseffected in response to pressurized nitrogen gas carried by first andsecond conduits 56 and 60. Gas flow through the conduits 56 and 60,typically at a maximum pressure of 60 psi, is regulated by conventionalelectrically actuated solenoid valves within a controller and valveactuation network 120. It is understood that pressurized nitrogen gas isallowed to flow through the conduits 56 and 60 when solenoid valves incommunication therewith are opened in response to electrical actuationby a controller 120.

Referring to FIG. 2, there is shown a cross-sectional side view of thereservoir 40. As shown in FIG. 2, the reservoir feed line 52 extendsinto a lower region of reservoir 40 from the valves 26 and 34. Thereservoir 40 will typically have a liquid storage capacity ofapproximately 700 ml., and in the preferred embodiment of FIG. 1includes a container defined by a bottom wall 80, a cylindrical sidewall84, and an upper wall 88. A pump feed line 92 leading to one or moredispensing pumps 96 extends through the bottom wall 80, and is therebyin liquid-flow communication with the interior of the reservoir 40. Highand low liquid-level sensors 98 and 99 are positioned on the sidewall84, and respectively provide signals indicative of whether the liquidlevel within the reservoir exceeds a full level or is less than a lowlevel. A redundant high-level (HI--HI) sensor 100 is positioned abovethe high level sensor 98, while a redundant low-level (LO--LO) sensor101 is positioned below the low liquid-level sensor 99.

The sensors 100 and 101 enhance safety and prevent contamination byensuring that the reservoir does not become either overfilled orcompletely empty. In particular, if the high liquid-level sensor 98 wereto become inoperative the HI--HI sensor 100 would provide a signal inthe event that the liquid within the reservoir 40 were to rise to thelevel thereof. In this way filling of the reservoir 40 would besuspended in a manner described hereinafter so as to prevent liquid fromentering a gas withdrawal conduit 102. Similarly, the LO--LO sensor 101is intended to signal when the reservoir 40 has become nearly empty inthe case of failure of the low liquid-level sensor 99. In this way theLO--LO sensor 101 serves to prevent air or gas from being introducedinto the pump feed line 92.

In a preferred implementation the HI--HI sensor 100 and LO--LO sensor101 are positioned in close proximity, e.g., within 10 mm., to the highand low liquid-level sensors 98 and 99, respectively. In addition, thereservoir 40 is designed in the preferred implementation such that avolume of approximately 35 ml. exists between the dispensing aperture103 of the reservoir feed line 52 and the intake port 104 of the pumpfeed line 92. This ensures that the reservoir 40 will not become emptyeven if the liquid level therein drops to the position of the LO--LOsensor 101. Moreover, since gas bubbles emerging from dispensingaperture 103 will tend to rise upon being introduced into the reservoir40, any such bubbles will be prevented from entering the pump feed line92 through the intake port 104 thereof. In the preferred implementationthe reservoir 40 is dimensioned so that:

a volume of approximately 60 ml. exists between the position of the lowliquid-level sensor 99 and the dispensing aperture 103,

a volume of approximately 450 ml. exists between the highland lowliquid-level sensors 98 and 99, and

the high liquid-level sensor 98 is separated from the upper wall 88 by avolume of approximately 90 ml.

Each of the sensors 98-101 may be implemented with, for example, acapacitive proximity sensor such as the Capacitive Proximity Sensor,Part No. E2K-F10MC2, manufactured by Omron Electronics, Inc. The sensorsmay be mounted external to the reservoir 40, and can detect the presenceof liquid therein without the requirement that apertures be formed inthe sidewall 84. The upper wall 88 includes an aperture through whichextends a gas withdrawal conduit 102 disposed to place the interior ofthe reservoir 40 in gas-flow communication with first three-way pressureactuated valve 105. The three-way valve 105 is pneumatically controlledwith pressurized gas, e.g., nitrogen, provided through pilot conduit108.

Referring to FIG. 1, a normally-closed port 107 of the three-way valve105 is in gas-flow communication with the vacuum port (V) of aconventional vacuum generator 110 such as is manufactured by Air Vac ofMilford, Conn., Part No. AVR-038H. The port 107 will be opened when gasis being withdrawn from the reservoir 40 by the vacuum generator 110,and will be closed otherwise. A common port 110 of the three-way valve105 is in continuous gas-flow communication with the interior of thereservoir 40 via the gas withdrawal conduit 102. When gas is not beingwithdrawn from the reservoir 40 through normally-closed port 107,nitrogen gas provided to a normally-open port 112 is bled into thereservoir 40 at slightly higher than atmospheric pressure through thecommon port 110.

The operation of the inventive dispensing system 10 may now beappreciated with reference to FIG. 1. In what follows it is to beremembered that the present invention contemplates that refilling of thereservoir 40 be carried out in accordance with the signals provided bythe liquid-level sensors 98 and 99, while changeover between thecontainers 14 and 18 be governed by the signals generated by the firstand second optical sensors 44 and 48.

Assume initially that the pumps 96 are withdrawing liquid from thereservoir 40, and that the reservoir 40 is concurrently beingreplenished by liquid from the first container 14. During this intervalliquid is drawn into the reservoir 40 as a consequence of the reducedpressure created therein by the vacuum generator 110. Under thesecircumstances the controller and valve actuation network 120 pressurizesthe conduit 108 so as to open the normally-closed port 107, and to closethe normally-open port 112, of the first three-way valve 105. Thisplaces the interior of the reservoir 40 in gas flow communication withthe vacuum port (V) of the vacuum generator 110 during the withdrawal ofliquid from the containers.

Liquid is withdrawn from the container 14 until the sensor 98 signalscontroller 120 that the liquid level within the reservoir 40 reaches areservoir full threshold. Controller 120 then electrically signals thatthe valve 64 and the port 107 be closed, which causes a suspension ingas pressurization of the conduits 56 and 108. This results in theclosure of the first pneumatically-controlled valve 26 and thenormally-closed port 107, thereby blocking liquid from container 14 fromentering the reservoir 40 and preventing vacuum generator 110 fromwithdrawing gas therefrom.

As the pumps 96 continue to remove liquid from the reservoir 40, theliquid level therein will decrease until it falls below the levelmonitored by the second sensor 99. At this point the second sensor 99signals the controller 120 that the reservoir 40 needs to be refilled.The controller 120 then initiates the refilling procedure in accordancewith the container preference specified by an operator (not shown). Forexample, if the operator (not shown) indicates to the controller 120that liquid from the first container 14 is to be used in replenishingthe reservoir 40 and the optical sensor 44 indicates that liquid ispresent in media supply line 22, then controller 120 issues theelectrical signals required to open the valve 26 and the normally-closedport 107, and to close the normally-open port 112. This inducespressurized nitrogen gas to flow through the conduits 56 and 108, andopens the pneumatically-controlled valve 26 and normally-closed port107. In this way the container 14 is placed in liquid-flow communicationwith the interior of the reservoir 40, thus allowing gas to be withdrawntherefrom by vacuum generator 110 in order to provide suction forpulling liquid from the container 14 into the reservoir 40.

As part of the refilling procedure the signals generated by theflow-through optical sensors 44 and 48 are monitored by the controller120 in order to determine whether liquid is available from either thefirst container 14 or the second container 18. If optical sensor 44signals that liquid is absent from supply line 22, then controller 120will be inhibited from pressurizing the conduit 56 and will provide analarm signal indicating that the container 14 is empty. Similarly, ifoptical sensor 48 signals that liquid is absent from supply line 30 thencontroller 120 will be unable to pressurize conduit 60 and will providean alarm signal indicating that the container 18 is empty.

Referring again to FIG. 1, the system 10 includes a source ofpressurized nitrogen (N₂) gas 140 in gas-flow communication with aconstant-pressure regulator and gauge 142. The gauge 142 will nominallybe set to a pressure of approximately 60 psi., and is connected to thealways-open common port 144 of a second three-way pressure actuatedvalve 150. Both the first and second three-way pressure actuated valves105 and 150 can be implemented with, for example, a 3-way Venturi valvesuch that manufactured Clippard of Cincinnati, Ohio, Part No. CLP-2012.When the reservoir 40 is not being refilled by either the container 14or by the container 18, the pressurized nitrogen provided by the source140 is delivered through a normally-open port 154 of the valve 150 to aconventional adjustable needle valve 156. In a preferred implementationthe needle valve 156 is set to approximately 3 psi., at a flow rate of 2liter/min. As shown in FIG. 1, the needle valve 156 is in gas-flowcommunication with an N₂ -gas filter 160 through a nitrogen supply line162. The filter 160 may be implemented with, for example, a 0.1 or 0.2μm gas filter such as is produced by Acrodisk of Ann Arbor, Michigan,Part No. GEL-4225.

As is indicated in FIG. 1, the nitrogen supply line 162 defines an N₂vent orifice 166 through which flows a portion of the N₂ gas stream. TheN₂ gas emanated by the orifice 166 bathes the system 10 in inertnitrogen gas, and hence reduces the likelihood of contamination bynon-inert gases within the surrounding atmosphere. The orifice 166further serves to fix the pressure of the N₂ gas provided by the filter160 to the normally-open port 112 at slightly higher than atmosphericpressure. This results in an N₂ gas stream being "bled" into theinterior of the reservoir 40 when it is not being refilled, i.e., whenit is isolated from the vacuum port (V) of vacuum generator 110 by thevalve 105.

In contrast, during refilling of the reservoir 40 the network 120pressurizes control conduit 170 so as to close the normally-open port154 and open a normally-closed port 174 of the three-way valve 150. Thisdirects the stream of N₂ gas from the regulator gauge 142 to a controlport (P) of the vacuum generator 110, which induces a vacuum at thevacuum port (V). Any nitrogen gas withdrawn from the reservoir 40 by thevacuum generator 110, together with the stream of nitrogen gas incidenton the control port (P), is dissipated through the exhaust port (E) ofthe vacuum generator 110.

FIGS. 3A-3D are a set of flow charts outlining the procedure by whichthe reservoir 40 is refilled in accordance with the present invention.As shown in FIG. 3A, the refill procedure is initiated by a reservoirextract routine 200. During the reservoir extract routine 200 a set ofstatus indicators are activated in accordance with the level of liquidin the reservoir 40. For example, in step 210 if it is determined viasensor 98 that the reservoir 40 is full then a status indicator isupdated in a step 212 to reflect that liquid is not being withdrawn fromthe liquid media container n selected by an operator. Similarly, if thesensor 100 indicates an overfill condition during step 220 thencontroller 120 activates the appropriate status indicator during a step240 and generates an alarm signal 244. If either of these conditionsexists then a reservoir extract latch within controller 120 is disabledin step 250. On the other hand, if it is ascertained that the reservoiris neither full nor overfilled a reservoir extract latch withincontroller 120 is set during step 252, a status indicator correspondingto a reservoir low condition is actuated 254, and the status indicatorassociated with container n is activated 256.

Referring to FIG. 3B, the reservoir extract routine 200 is followed by adispense mode select procedure 260 in which it is determined if thesystem 10 is disposed to be placed in an auto dispense mode. The supplylines 22 and 30 are inspected in an initial step 270 to determine ifliquid media is present in the containers 14 and 18. If not, theoperator is alerted via generation of a container empty alarm signal280. Otherwise, dispensing from the selected container is allowed toproceed following the setting of an auto mode enable latch in step 312.

As shown in FIG. 3C, the dispense mode select procedure is followed by amedia select and control procedure 320 during which liquid is dispensedfrom the container selected by the operator in the manner describedabove. For example, in steps 330 and 340 the signals from theflow-through sensors 44 and 48 are analyzed by controller 120 todetermine if liquid media, i.e., MEDIA A and MEDIA B, is present withinthe first and second containers 14 and 18. If so, the appropriate statusindicators are updated in steps 350 and 360 to reflect that dispensingmay be carried out using MEDIA A or MEDIA B. If not, media empty latcheswithin controller 120 are set in steps 370 and 380 to prevent dispensingfrom occurring from an empty container.

If fluid is determined to be present in the containers 14 and 18 duringthe steps 330 and 340, then controller 120 is interrogated during steps420 and 430 in order to determine from which of the containers liquid isto be dispensed. Next, the media select latch within controller 120corresponding to the container selected by the operator is set duringsteps 440 and 450. If gas is detected in the media supply lines 22 and30 (FIG. 1) by the sensors 44 and 48 during the steps 330 and 340, thenMEDIA A and MEDIA B empty alarms are actuated in step 470.

Referring to FIG. 3D, the reservoir 40 is filled in accordance with thesteps included within a reservoir fill procedure 500. In steps 505 and510 it is determined whether the first or second container 14 or 18 hasbeen selected to serve as the source of liquid for the reservoir 40.Replenishing of the reservoir does not commence, however, until thereservoir extract latch, the auto mode select latch, and the mediaselect latch for the selected container have been set in steps 515, 520and 525. In addition, it is determined whether the reservoir empty bithas been reset in step 530. Filling of the reservoir is then initiatedin step 540 or step 542 if the reservoir 40 has been found not to befull (step 550), or overfilled (step 545).

While the present invention has been described with reference to a fewspecific embodiments, the description is illustrative of the inventionand is not to be construed as limiting the invention. For example,although the preferred embodiment of FIG. 1 contemplates automaticswitching between a pair of containers during refilling of thereservoir, alternative embodiments may employ only a singledirect-dispense container. In such alternative embodiments the mediasupply line extending into the container would preferably be opticallymonitored, with refill of the reservoir being suspended if it isdetermined that gas is present therein, i.e., that the container hasbecome empty. Various other modifications may occur to those skilled inthe art without departing from the true spirit and scope of theinvention as defined by the appended claims.

What is claimed is:
 1. A system for dispensing liquid from first andsecond containers, comprising:first media transfer line means,positioned in liquid-flow communication with said first container, fortransferring liquid from said first container; second media transferline means, positioned in liquid-flow communication with said secondcontainer, for transferring liquid from said second container;switchable valve means for, in a first state, preventing liquid fromflowing through said first media transfer line means while allowingliquid to flow through said second media transfer line means and for, ina second state, preventing liquid from flowing through said second mediatransfer line means while allowing liquid to flow through said firstmedia transfer line means: a liquid storage reservoir connected inliquid-flow communication with said valve means for storing liquidreceived from said first and second media transfer line means by way ofsaid valve means; bleeding valve means for selectively introducing gasinto said liquid storage reservoir; pump means for withdrawing liquidstored within said liquid storage reservoir; transfer line sensor meansfor monitoring said first and second media transfer line means for thepresence of liquid therein, and for generating a first switching signalwhen liquid becomes absent from said first media transfer line and asecond switching signal when liquid becomes absent from said secondmedia transfer line; container switching means for alternately placingsaid first and second containers in liquid-flow communication with saidreservoir by switching said valve means to said first state in responseto said first switching signal and by switching said valve means to saidsecond state in response to said second switching signal; and means forinducing liquid to flow from said transfer line means into said liquidstorage reservoir.
 2. The system of claim 1 wherein said transfer linesensor means includes a first optical sensor for providing said firstswitching signal and a second optical sensor for providing said secondswitching signal, said first optical sensor being in optical alignmentwith said first media transfer line means and said second optical sensorbeing in optical alignment with said second media transfer line means.3. The system of claim 1 wherein said liquid comprises a liquidchemical, said system further including a source of inert gas inswitchable gas-flow communication, through said bleeding valve means,with said liquid storage reservoir.
 4. The system of claim 1 whereinsaid switchable valve means includes a first valve influid-communication with said first media transfer line and a secondvalve in fluid-communication with said second media transfer line, saidfirst and second valves being respectively in closed and open positionswhen said valve means is in said first state and being respectively inopen and closed positions when said valve means is in said second state.5. The system of claim 4 wherein said first and second valves comprisegas-actuated pneumatic valves, and wherein said switching means includespilot valve means for directing compressed gas to said first and secondvalves in order to switch said first and second valves to said open andclosed positions.
 6. The system of claim 1 further including levelsensing means for monitoring the level of liquid within said storagereservoir, and for providing a reservoir low signal when the liquidlevel within said reservoir falls below a first predetermined level. 7.The system of claim 6 wherein said means for inducing liquid to flowcomprises a vacuum generator, said system further including firstthree-way valve means for placing said vacuum generator in gas-flowcommunication with said reservoir means upon receiving said reservoirlow signal from said level sensing means.
 8. The system of claim 7wherein said level sensing means includes a first low level sensormounted at a first predetermined location on said reservoir forgenerating said reservoir low signal, and a first high level sensormounted at a second predetermined location on said reservoir forgenerating a reservoir high signal when the liquid level within saidreservoir rises above a second predetermined level.
 9. The system ofclaim 8 wherein said first three-way valve means is operative to blockgas-flow communication between said reservoir means and said vacuumgenerator upon receiving said reservoir high signal from said levelsensing means.
 10. The system of claim 8 wherein said switchable valvemeans includes means for placing said first and second valves in saidclosed position upon receiving said reservoir high signal from saidlevel sensing means.
 11. The system of claim 8 wherein said levelsensing means includes a second high level sensor mounted at apredetermined location on said reservoir above said first high levelsensor, for generating an overfill signal when the level of liquidwithin said reservoir rises above a predetermined overfill level.
 12. Asystem for dispensing liquid from first and second containers,comprising:first media transfer line means, positioned in liquid-flowcommunication with said first container, for transferring liquid fromsaid first container; second media transfer line means, positioned inliquid-flow communication with said second container, for transferringliquid from said second container; switchable valve means for, in afirst state, preventing liquid from flowing through said first mediatransfer line means while allowing liquid to flow through said secondmedia transfer line means and for, in a second state, preventing liquidfrom flowing through said second media transfer line means whileallowing liquid to flow through said first media transfer line means: aliquid storage reservoir connected in liquid-flow communication withsaid valve means for storing liquid received from said first and secondmedia transfer line means by way of said valve means; level sensingmeans for monitoring the level of liquid within said storage reservoir,and for providing a reservoir low signal when the liquid level withinsaid reservoir falls below a first predetermined level; pump means forwithdrawing liquid stored within said liquid storage reservoir; transferline sensor means for monitoring said first and second media transferline means for the presence of liquid therein, and for generating afirst switching signal when liquid becomes absent from said first mediatransfer line and a second switching signal when liquid becomes absentfrom said second media transfer line; valve switching means forswitching said valve means to said first state in response to said firstswitching signal and for switching said valve means to said second statein response to said second switching signal; means for inducing liquidto flow from said transfer line means into said liquid storagereservoir, said means for inducing liquid to flow comprising a vacuumgenerator; first three-way valve means for placing said vacuum generatorin gas-flow communication with said reservoir means upon receiving saidreservoir low signal from said level sensing means; and gas supply meansfor bleeding gas into said reservoir means through said first three-wayvalve means when gas-flow communication is blocked between saidreservoir means and said vacuum generator.
 13. The system of claim 12wherein said gas supply means includes second three-way valve means ingas-flow communication with a control port of said vacuum generator andwith said first three-way valve means.
 14. A system for dispensingliquid media from first and second containers comprising:means forselectively transferring said media from either said first containerthrough a first media transfer line, or from said second containerthrough a second media transfer line, to a liquid storage reservoir;means for monitoring said transfer of media through said media transferlines and switching liquid flow between said containers into saidreservoir when media is not being transferred through one of said mediatransfer lines; means for selectively introducing gas into said liquidstorage reservoir; and means for selectively withdrawing said gas fromwithin said liquid storage reservoir: wherein said transfer of mediathrough said transfer lines is expedited by said selective withdrawal ofgas therefrom.
 15. A method of dispensing liquid media from first andsecond containers comprising the steps of:selectively transferring saidmedia from either said first container through a first media transferline, or from said second container through a second media transferline, to a liquid storage reservoir; monitoring said transfer of mediathrough said media transfer lines and switching liquid flow between saidcontainers into said reservoir when media is not being transferredthrough one of said media transfer lines; and selectively introducinggas into said liquid storage reservoir: and selectively withdrawing gasfrom within said liquid storage reservoir in order to induce liquid toflow from said first and second media transfer lines into saidreservoir.
 16. A method for dispensing liquid from first and secondcontainers, comprising the steps of:transferring liquid from said firstcontainer through a first media transfer line into a liquid storagereservoir; transferring liquid from said second container through asecond media transfer line into said liquid storage reservoir;withdrawing liquid stored within said liquid storage reservoir;selectively introducing gas into said liquid storage reservoir;selectively withdrawing said gas from within said liquid storagereservoir in order to induce liquid to flow from said first and secondmedia transfer lines into said reservoir; generating a first switchingsignal when liquid becomes absent from said first media transfer lineand a second switching signal when liquid becomes absent from saidsecond media transfer line; preventing liquid from flowing in said firstmedia transfer line while allowing liquid to flow through said secondmedia transfer line in response to said first switching signal, andpreventing liquid from flowing in said second media transfer line whileallowing liquid to flow in said first media transfer line in response tosaid second switching signal.
 17. The method of claim 16 furtherincluding the steps of:monitoring the level of liquid within saidstorage reservoir, and providing a reservoir low signal when the liquidlevel within said reservoir falls below a first predetermined level. 18.The method of claim 17 wherein said liquid comprises a liquid chemical.19. A system for dispensing liquid from first and second containers,comprising:first media transfer line means, positioned in liquid-flowcommunication with said first container, for transferring liquid fromsaid first container; second media transfer line means, positioned inliquid-flow communication with said second container, for transferringliquid from said second container; switchable valve means for, in afirst state, preventing liquid from flowing through said first mediatransfer line means while allowing liquid to flow through said secondmedia transfer line means and for, in a second state, preventing liquidfrom flowing through said second media transfer line means whileallowing liquid to flow through said first media transfer line means; aliquid storage reservoir connected in liquid-flow communication withsaid valve means for storing liquid received from said first and secondmedia transfer line means by way of said valve means; level sensingmeans for monitoring the level of liquid within said storage reservoir,and for providing a reservoir low signal when the liquid level withinsaid reservoir falls below a first predetermined level, said levelsensing means including a first low level sensor mounted at a firstpredetermined location on said reservoir for generating said reservoirlow signal, and a first high level sensor mounted at a secondpredetermined location on said reservoir for generating a reservoir highsignal when the liquid level within said reservoir rises above a secondpredetermined level wherein said level sensing means further includes asecond high level sensor mounted at a predetermined location on saidreservoir above said first high level sensor, for generating an overfillsignal when the level of liquid within said reservoir rises above apredetermined overfill level; pump means for withdrawing liquid storedwithin said liquid storage reservoir; transfer line sensor means formonitoring said first and second media transfer line means for thepresence of liquid therein, and for generating a first switching signalwhen liquid becomes absent from said first media transfer line and asecond switching signal when liquid becomes absent from said secondmedia transfer line; valve switching means for switching said valvemeans to said first state in response to said first switching signal andfor switching said valve means to said second state in response to saidsecond switching signal; means for inducing liquid to flow from saidtransfer line means into said liquid storage reservoir, said means forinducing liquid to flow comprising a vacuum generator; and three-wayvalve means for placing said vacuum generator in gas-flow communicationwith said reservoir means upon receiving said reservoir low signal fromsaid level sensing means, said three-way valve means being operative toblock gas-flow communication between said reservoir means and saidvacuum generator upon receiving said overfill signal from said levelsensing means.
 20. A system for dispensing liquid from first and secondcontainers, comprising:a first media transfer line, positioned inliquid-flow communication with said first container, for transferringliquid from said first container; a second media transfer line,positioned in liquid-flow communication with said second container, fortransferring liquid from said second container; a switchable valve for,in a first state, preventing liquid from flowing through said firstmedia transfer line while allowing liquid to flow through said secondmedia transfer line and for, in a second state, preventing liquid fromflowing through said second media transfer line means while allowingliquid to flow through said first media transfer line; a liquid storagereservoir connected in liquid-flow communication with said switchablevalve for storing liquid received from said first and second mediatransfer lines by way of said switchable valve; a source of inert gas inswitchable gas-flow communication with said liquid storage reservoir; apump for withdrawing liquid stored within said liquid storage reservoir;a transfer line sensor arrangement for monitoring said first and secondmedia transfer lines for the presence of liquid therein, and forgenerating a first switching signal when liquid becomes absent from saidfirst media transfer line and a second switching signal when liquidbecomes absent from said second media transfer line; valve switchingmeans for switching said switchable valve to said first state inresponse to said first switching signal and for switching saidswitchable valve to said second state in response to said secondswitching signal; and a vacuum generator for inducing liquid to flowfrom said first and second transfer lines into said liquid storagereservoir.
 21. The system of claim 20 further including a first valvefor placing said source of inert gas in gas-flow communication with saidliquid storage reservoir when said liquid stored within said storagereservoir is not being withdrawn from said reservoir.
 22. The system ofclaim 20 further including a first valve for placing said source ofinert gas in gas-flow communication with said liquid storage reservoirwhen said vacuum generator is not operating to induce said liquid toflow from said first and second transfer lines into said liquid storagereservoir.
 23. A system for dispensing liquid from first and secondcontainers, comprising:a first media transfer line, positioned inliquid-flow communication with said first container, for transferringliquid from said first container; a second media transfer line,positioned in liquid-flow communication with said second container, fortransferring liquid from said second container; a switchable valve for,in a first state, preventing liquid from flowing through said firstmedia transfer line while allowing liquid to flow through said secondmedia transfer line and for, in a second state, preventing liquid fromflowing through said second media transfer line while allowing liquid toflow through said first media transfer line; a liquid storage reservoirconnected in liquid-flow communication with said switchable valve forstoring liquid received from said first and second media transfer linesby way of said switchable valve; a source of inert gas in switchablegas-flow communication with said liquid storage reservoir reservoirlevel sensor means for monitoring the level of said liquid stored withinsaid reservoir; a pump for withdrawing liquid stored within said liquidstorage reservoir; container switching means for switching saidswitchable valve to said first state in response to removal ofsubstantially all of said liquid from said first container, and forswitching said switchable valve to said second state in response toremoval of substantially all of said liquid from said second container;and a vacuum generator, responsive to said reservoir level sensor means,for inducing liquid to flow from said first and second transfer linesinto said liquid storage reservoir in order to maintain the level ofsaid liquid stored within said reservoir between upper and lowerreservoir fill levels.
 24. The system of claim 23 wherein said containerswitching means includes:a transfer line sensor arrangement formonitoring said first and second media transfer lines for the presenceof liquid therein, and for generating a first switching signal whenliquid becomes absent from said first media transfer line and a secondswitching signal when liquid becomes absent from said second mediatransfer line, and valve switching means for switching said switchablevalve to said first state in response to said first switching signal andfor switching said switchable valve to said second state in response tosaid second switching signal.
 25. The system of claim 23 wherein saidreservoir level sensor means includes upper and lower sensors disposedat said upper and lower fill levels of said reservoir, said upper andlower fill levels being selected such that the volume within saidreservoir therebetween substantially exceeds the volume within saidreservoir below said lower fill level.